Many electrical circuits are “multiple voltage systems”, in the sense that they require multiple supply voltages of differing voltage levels. These applications typically require a controlled application of power during power-up and power-down.
Controlled sequencing, with selected voltages applied before others during power up, has been used to solve some problems that arise in multiple voltage systems. DSPs and other multi-voltage microprocessors usually require their I/O voltage to be present before applying the core voltage, or vice versa. Another situation that requires sequencing is a board that includes a secondary controller like a graphics controller along with the main CPU. To avoid uncontrolled outputs on the graphic display, the CPU must be up and running before the graphics controller receives power.
Often, however, it is voltage levels rather than a particular sequence of voltage application that cause problems. In particular, today's more complicated semiconductor chips and chip sets often operate at more than one voltage level. Groups of pins may each operate at a different voltage level. To prevent latch-up, one approach has been to control the sequence in which supply voltages are applied to the chip. In general, controlled power sequencing is critical to the operation of many computer-oriented systems.
As a specific example, for a semiconductor circuit that operates on bipolar supply voltages, the most positive voltage can be applied first, then a logic supply, and the negative supply voltage last. Violating this rule can cause latch-up within the semiconductor. Analog circuits, such as multiplexers, can also have latch-up problems.